Systems and methods for retransmission rate selection to support fast moving devices

ABSTRACT

The present disclosure is directed generally to systems and methods for dynamically selecting a retransmission rate for communication between two devices. The retransmission rate may be selected after a transmission has failed between the devices. In some embodiments, the retransmission rate may be selected based on relative velocity of the devices. In some embodiments, the retransmission rate may be selected based on transmission channel characteristics. The retransmission rate may be selected such that the retransmission will be less than an allowable transmission rate based on a distance between the two devices.

FIELD

The present application generally relates to data communication networksand devices. In particular, the present application relates to systemsand methods for retransmission rate selection for wirelesscommunication.

BACKGROUND

A device may act as an access point that connects a client device to anetwork wirelessly. The access point may communicate with the device ata transmission rate. A higher transmission rate may provide greaterbandwidth than a lower transmission rate but may also have a shorterrange. Signals between the access point and the client device may bedegraded by various factors, such as physical obstruction, interference,or distance. Due to signal degradation, some transmissions may fail andneed to be retransmitted.

BRIEF SUMMARY

In wireless local area network (WLAN), data transmissions between twodevices can make use of multiple physical data rates or transmissionrates. The two devices can be access point (AP) to Client in aninfrastructure network or AP-to-AP in a peer-to-peer network, etc. Whilea higher physical data rate provides better data throughput, it requiresa higher signal-to-noise ratio (SNR) to work. Rate adaptation is amechanism for a wireless device to adapt its physical data transmissionrate based on the perceived channel condition. Due to wireless channelcondition fluctuations, such as shadowing, interference, or noise, apacket transmission using a first selected high physical data rate mayfail. To increase the chance that the packet can be deliveredsuccessfully, a number of retransmissions using different (typicallylower) physical data rates are used.

Rate adaptation is designed to select data rates for transmissions toachieve the highest data throughput. Therefore, the first selected datarate for a packet transmission is usually the highest data rate possiblefor a given environment and the retransmission data rates are usuallythe immediate lower data rates (second highest, third highest, and soon). These immediate lower data rates are usually quite similar in termsof SNR requirements to the first selected data rate for the receiver tosuccessfully decode the transmission. This scheme usually is suitable inscenarios where the two communicating devices are stationary, such thatreceived signal strength from each device remains approximatelyconstant.

However, if one or both devices are moving quickly, their distancebetween them can change very quickly over time (e.g., train totrackside, car to stationary access point, plane to tower communication,etc.). This fast change in the channel conditions between the twocommunicating devices may cause the first selected data rate to fail.The situation could be even worse if the two communicating devices aremoving away from each other; in this case, the devices will be separatedeven further apart from each other after the transmission failures usingthe first transmission rate. Even if the immediate lower data rate willbe used for retransmissions, the channel conditions between the twodevices could be so degraded that the transmissions using these datarates will likely to fail as well. This could potentially causeincreased latency, packet loss, and/or reduced throughput of thewireless connection.

In order to achieve more reliable data communications for fast movingwireless devices, in some implementations of the methods and systemsdiscussed herein, a rate adaptation mechanism may select a more reliableand yet fastest allowable transmission rates for retransmissions. Whenselecting the retransmission rates for a packet transmission, the SNRrequirements for the retransmission rates will be substantially lowerthan the previous transmission rate (for example, a SNR differencebetween the 1st rate and the 2nd rate may be less than a SNR differencebetween the 2nd rate and 3rd rate, and so on) so that the differences inthe SNR requirement of the current retransmission rate can withstand aworst-case degradation in SNR due to the devices moving away from eachother. This increases the chance of successful packet delivery as wellas reducing the total packet transmission time, and thus increases theoverall throughput of the connection. In some implementations, thedifference in SNR between the retransmission rates may be determinedbased on the relative speed of the moving devices in someimplementations. In other implementations, the difference in SNR betweenthe retransmission rates may be determined based on observed or measuredpast receive signal strength fluctuations.

The present application is directed to methods and systems for selectingretransmission rates to support fast moving devices in wirelesscommunication, such as in WLAN. In one aspect, the present applicationis directed to selecting a retransmission rate to account for therelative velocity of the devices transmitting after a transmission froman access point to a client device fails. Accounting for the relativevelocity or past received signal strength fluctuations of the devicesallows for better estimates of possible transmission rates, thusincreasing the likelihood that the retransmission will succeed.

At least one aspect of the present disclosure is directed to a methodfor wireless retransmission rate selection. In some embodiments, themethod includes transmitting, by a first device to a second device, adata packet at a first rate. The method can include determining, by thefirst device, to retransmit the data packet. The method can furtherinclude identifying, by the first device, a relative velocity betweenthe first device and second device. The method can also includeretransmitting the data packet, by the first device to the seconddevice, at a second rate selected responsive to the identified relativevelocity or past received signal strength fluctuations.

In some embodiments, the method includes selecting the second rate froma plurality of potential transmission rates, each of the plurality ofpotential transmission rates corresponding to a required value for acharacteristic of a communication channel between the first device andthe second device. In some embodiments, the method includes monitoringthe characteristic of the communication channel between the first deviceand second device, and determining that a monitored characteristic ischanging at a rate exceeding a predetermined threshold. In someembodiments, the method includes estimating a value for the monitoredcharacteristic at a time period subsequent to a retransmission attempt,based on the rate of change of the monitored characteristic, andselecting a fastest transmission rate of the plurality of potentialtransmission rates having a corresponding required value for thecharacteristic less stringent than the estimated value.

In some embodiments of the method, the characteristic comprises aminimum signal to noise ratio of the communication channel. In someembodiments of the method, the characteristic comprises a minimummeasured signal strength of a transmission by the second device. In someembodiments of the method, the characteristic comprises a maximum blockerror rate of the communication channel.

In some embodiments, the method includes monitoring an acceleration ofthe first device or a change in position of the first device from anidentified stationary position of the first device. In some embodiments,the method includes selecting the second rate from a plurality ofpotential transmission rates, each of the plurality of potentialtransmission rates corresponding to a minimum acceleration of the firstdevice between the first device and the second device.

At least one aspect of the present disclosure is directed to anothermethod for wireless retransmission rate selection. In some embodiments,the method includes transmitting, by a first device to a second device,a data packet at a first rate having a first required value for atransmission channel characteristic and determining, by the firstdevice, to retransmit the data packet. The method further includesretransmitting the data packet, by the first device to the seconddevice, at a second rate having a second required value for thetransmission channel characteristic, and determining, by the firstdevice, to retransmit the data packet again. The method further includesselecting, by the first device, a third rate, and retransmitting thedata packet, by the first device to the second device, at the thirdrate.

In some embodiments, the third rate is selected such that a differencebetween a third required value of the third rate for the transmissionchannel characteristic and the second required value is greater than adifference between the second required value and the first requiredvalue. In some embodiments of the method, selecting the third rate isperformed responsive to identifying, by the first device, a relativevelocity between the first device and second device exceeding apredetermined threshold. In some embodiments, the method includesselecting the third rate such that the difference between the thirdrequired value and the second required value is greater than adifference between the second required value and the first requiredvalue multiplied by a factor proportional to the relative velocity.

In some embodiments of the method, determining to retransmit the packetand determining to retransmit the data packet again are each performedresponsive to an expiration of a retransmission time out period for theprevious transmission of the packet. In some embodiments of the method,determining to retransmit the packet and determining to retransmit thedata packet again are each performed responsive to not receiving aresponse to the packet within a predetermined time period. In someembodiments of the method, the predetermined time period is based on anaverage time to receive a response for one or more previouslytransmitted packets.

In some embodiments of the method, the transmission channelcharacteristic comprises a minimum signal to noise (SNR) requirement. Insome embodiments of the method, the transmission channel characteristiccomprises a signal strength of packets received from the second device.In some embodiments of the method, the transmission channelcharacteristic comprises a maximum block error rate.

At least one aspect is directed to a system for wireless retransmissionrate selection. In some embodiments, the system can include a firstdevice. In some embodiments, the first device can include a networkinterface and a processor executing a transmission rate selector. Insome embodiments, the network interface is configured to transmit a datapacket from the first device to a second device at a first rate,retransmit the data packet from the device to a second device at asecond rate, and retransmit the data packet from the first device to thesecond device at a third rate. In some embodiments, the transmissionrate selector is configured to select the third rate such that adifference between a required value of a transmission channelcharacteristic for the first rate and a required value of thetransmission channel characteristic for the second rate is less than adifference between the required value of the transmission channelcharacteristic for the second rate and a required value of thetransmission channel for the third rate.

In some embodiments of the system, the transmission rate selector isfurther configured to identify a relative velocity between the firstdevice and the second device, and wherein the third rate is selectedsuch that the difference between the required value of the transmissionchannel characteristic for the first rate and the required value of thetransmission channel characteristic for the second rate is less than thedifference between the required value of the transmission channelcharacteristic for the second rate and the required value of thetransmission channel for the third rate multiplied by a factorproportional to the relative velocity. In other embodiments, pastmeasurements of received signal strength or fluctuations in measuredreceived signal strength may be used to determine the third rate.

The details of various embodiments of the disclosure are set forth inthe accompanying drawings and the description below.

BRIEF DESCRIPTION OF THE FIGURES

The foregoing and other objects, aspects, features, and advantages ofthis disclosure will become more apparent and better understood byreferring to the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1A is a diagrammatic view of an embodiment of an access point and anon-stationary client;

FIG. 1B is a diagrammatic view of an embodiment of a non-stationaryaccess point, a stationary first client and a non-stationary secondclient;

FIG. 2 is a flowchart of an embodiment of a method of stepwiseretransmission rate selection;

FIG. 3 is a graph depicting an embodiment of a timeline withtransmission rates using the dynamic transmission selection of FIG. 2;

FIG. 4 is a flowchart of an embodiment of a method of dynamicallyselecting transmission rate responsive to monitoring a communication;

FIG. 5 is a flowchart of an embodiment of a method of dynamicallyselecting transmission rate responsive to differences in transmissionrates;

FIG. 6A is a graph depicting an embodiment of a timeline withtransmission rates using one implementation of dynamic transmission rateselection;

FIG. 6B is a graph depicting an embodiment of a timeline withtransmission rates using another implementation of dynamic transmissionrate selection;

FIG. 6C is a graph depicting another embodiment of a timeline withtransmission rates using another implementation of dynamic transmissionrate selection;

FIG. 7 is a set of graphs depicting embodiments of timelines thatillustrate the illustrate the relationship between time, distance toaccess point, signal to noise ratio, and transmission rate possible; and

FIG. 8 is a block diagram illustrative of an embodiment of a system fordynamic transmission rate selection.

The features and advantages of the present disclosure will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements.

DETAILED DESCRIPTION

FIG. 1A is a diagrammatic view of an embodiment of an access point and anon-stationary client. In wireless communication, an access point 101may connect one or more clients 103 to a network, such as a local areanetwork (LAN), WLAN, or the Internet. In some embodiments, both theaccess point 101 and the client 103 may be stationary or mostlystationary. For example, an access point 101 may be a wireless routerand a client 103 may be a desktop computer. In another example, theclient 103 may be a laptop computer or a mobile device that does notmove outside the range of the access point 101 that uses a specifictransmission rate. In some embodiments, both the access point 101 andthe client 103 may be moving together, e.g. in a same vehicle, such thatthey are not moving relative to each other. In many such embodiments,retransmissions may be made at the same transmission rate as originaltransmission attempts, as signal strengths and noise levels may remainroughly constant.

In some embodiments, the client may be moving relative to each other.For example, the access point may be located in a train station and theclient 103 may be located in a train that at first approaches closer tothe train station 103′ then travels away from the station 103″ (client103, 103′, and 103″ accordingly depict the same client in differentpositions). In another example, the client 103 may be a mobile devicethat may be inside of a moving automobile. The access point 101 may be adata network provider capable of selecting a transmission rate with aclient 103.

When the client 103 is moving relative to the access point 101, atransmission between the access point 101 and the client 103 may fail.For example, the client 103″ may be moving away from the access point101 such that the transmission rate used cannot cover the increaseddistance between the devices, as signal strength declines proportionalto the distance between transmitter and receiver. In another example,the client 103 may move behind an obstacle or receive interference froman antenna that prevents the transmission from succeeding at aparticular transmission rate. In some instances, the transmission ratemay be set at a lower rate than what is possible, as when the client 103is approaching the access point 101, because the transmission rate isset when the client 103 is far away but is not changed even when theclient 103 is closer 103′ to the access point 101.

FIG. 1B is a diagrammatic view of an embodiment of a non-stationaryaccess point 104, a stationary first client 105 and a non-stationarysecond client 106. In some embodiments, the access point 104 may be in adrone or a moving vehicle such as a balloon, car, train, or an airplane.In some embodiments, the access point 104 may be a mobile phone thatcreates a WiFi hotspot. A first client 105 may be stationary and theaccess point 104 may at first move closer 104′ to the first client 105and then move away from the first client 105 (access point 104, 104′,and 104″ accordingly depict the same access point in differentpositions). Because the access point 104 may be moving, transmissionrate possible between the access point 104 and the first client 105 mayincrease as the access point moves closer 104′ to the first client 105.In other instances, the transmission may fail due to the access point104 moving farther away 104″ from the first client 105. In someinstances, clients may also be moving. For instance, a second client 106may be moving in a different direction compared to the access point 104(second client 106, 106′, and 106″ accordingly depict the same secondclient in different positions). Thus, the transmission rate possible mayincrease dramatically then drop off dramatically.

Referring now to FIG. 2, illustrated is a flowchart of an embodiment ofa method of dynamically selecting retransmission rates. In briefoverview, at step 201, a device may select an initial transmission (TX)rate. At step 202, the device may transmit at a selected rate. At step203, the device may determine whether the transmission failed. If thetransmission succeeds, the device may not need to perform anyretransmissions, and the method may end or repeat for subsequent packetsor data until a transmission fails or the connection is closed. If thetransmission fails at step 203, then at step 204, the device may selecta lower TX rate. The device may try to retransmit the data at the lowertransmit rate at step 205. In some implementations, steps 203-205 may berepeated iteratively until successfully transmitting the data. In otherimplementations, the device may maintain a retransmission counter ortimer, and may repeat steps 203-205 up to a predetermined number oftimes according to the counter or for a predetermined time according tothe timer until reaching a retransmission timeout or successfullycompleting transmission.

Still referring to FIG. 2, and in more detail, at step 201, the devicemay select an initial TX rate. The device may be referred to as a firstdevice. In some embodiments, the device may be an access point or aclient device as described in FIGS. 1A and 1B. The initial TX rate maybe a maximum TX rate of the device, which may have a lower range orphysical distance that the signal can be transmitted compared to a lowerTX rate. The maximum TX rate may be set based on the specifications ofthe device.

At step 202, the device may attempt to transmit data to another deviceat a selected rate. The other device may be referred to as a recipientdevice. For instance, after step 201, an initial TX rate may be theselected TX rate. In some embodiments where the device is an accesspoint, the access point may attempt to send a packet of data to a clientdevice at the initial TX rate.

At step 203, the device may determine whether the transmission to theother device has failed. In some embodiments where the device is anaccess point, the access point may wait for a response from the clientdevice after sending the transmission at the selected rate. After apredefined period of time (e.g. a retransmission timeout or RTO, used bymany lossless transmission protocols), the access point may not receivea reply, indicating that the transmission has failed. In some instances,the access point may receive a reply that comprises too much noise suchthat data in a received packet cannot be read. In some instances, theaccess point may receive a reply from the client device indicating thatthe transmission has failed (e.g. a negative acknowledgement or NACK).If the transmission has not failed, then another transmission may besent to the other device at the same selected rate. In a furtherimplementation, a device may retransmit a packet several times withoutwaiting for expiration of a retransmission timeout or receipt of a NACK,for example, responsive to detection of high noise levels or increasingnoise levels on a transmission channel, or responsive to detection of afast relative velocity between the device and recipient device. This maybe particularly useful in instances in which a retransmission timeoutmay be very long in comparison to the time in which a device is withintransmission range of the other device (e.g. an RTO of several secondsand a device on a train passing a train platform at 50 miles per hour).If the transmission succeeds, then the method may be repeated foradditional packets or data, or until the connection is closed.

At step 204, if the transmission has failed, the device selects a lowerTX rate to retransmit the failed transmission. The device may thenretransmit the data or packet at step 205. The lower TX rate may belower than the TX rate of the failed transmission. The lower TX rate mayhave a longer range than a higher TX rate of the failed transmission. Ifthe retransmission fails again at step 205, an even lower TX rate may beselected at step 204. In some instances, a lower TX rate may be selectedbased on a stepwise function. Differences between retransmission ratesmay be the same (e.g. 250 Mbps, 200 Mbps, 150 Mbps, etc.), or different(e.g. 250 Mbps, 200 Mbps, 100 Mbps, 20 Mbps, etc.), in variousimplementations.

FIG. 3 is a graph depicting an embodiment of a timeline withtransmission rates selected using a stepwise transmission rate selectionmethod. When the distance between a first device and a second device ischanging, then the allowable TX rate will vary over time due to thevarying distance between the devices. For instance at time 301, the twodevices may start relatively close to one another, and thus the maximumallowable TX rate based on the distance may be relatively high at 321.In some instances, the devices may move away from each other, and themaximum allowable TX rate may decrease: at time 302, for instance, theallowable TX rate may decrease to 322; at time 303, the allowable TXrate may yet again decrease to 323; and at time 304, the allowable TXrate may yet again decrease to 324.

Using one implementation of the method described in relation to theflowchart of FIG. 2, the devices may initially attempt to transmit at ahigher rate than the maximum allowable rate, and thus the transmissionmay fail. For instance, at time 301, the TX rate 311 may be greater thanthe allowable TX rate 321. In some instances, at an early point in time,the access point may have successfully transmitted to the client deviceat the TX rate of 311, and so the access point may continue to attemptto transmit at that rate. In other instances, when the access point isconnecting to a new client device, the access point may attempt totransmit at a high transmit rate 311, which may be the highest TX ratepossible on the access point. However, at time 301, because the TX rate311 is greater than the allowable TX rate 321, the transmission mayfail. At the next time increment 302, the access point may attempt alower TX rate 312 to retransmit the failed transmission. However, due tothe increased distance between the devices, the allowable TX rate 322also decreased, and the retransmission fails again. This step may repeatat times 303 and 304 and with the TX rates tried by the device at 313and 314. As the distance between the devices may be continuallyincreasing, each subsequent attempt to retransmit may fail, when at someparticular points in time such as at 302, 303, and 304, a lower TX ratemay have succeeded.

In many implementations, a retransmission timeout may be very short ormay be too brief to allow a device to monitor channel conditions orobtain feedback regarding channel characteristics in betweenretransmissions of the same packet or data. In some suchimplementations, the device may periodically or continuously monitorchannel characteristics (such as measuring received signal strength fromthe destination device, measuring noise levels, etc.). This may be donevia monitoring of received data packets or by a periodic data exchange(e.g. ping, null data, etc.) in the absence of regular data traffic.Changes or fluctuations in these channel characteristics may be used tomake predictions or estimations of future characteristics, andretransmission rates may be selected accordingly. FIG. 4 is a flowchartof an embodiment of a method of dynamically selecting transmission rateresponsive to monitoring a communication. In brief overview, at step405, a device may select an initial transmission rate. At step 410, thedevice may transmit to a second device at the transmission rate. At step415, the device may determine whether the transmission has failed. Ifthe transmission has succeeded, then the device may transmit anotherpacket of data to the second device at the same transmission rate atstep 410. At step 416, if the transmission has failed, then the devicemay identify a relative velocity between the device and the seconddevice. Identifying the relative velocity may comprise the devicemonitoring a characteristic of a communication channel of thetransmission at step 420, and the device determining whether thecharacteristic is changing at a rate exceeding a threshold at step 430.If the characteristic is changing at a rate below the threshold, in someimplementations, the device retransmits at the previous transmissionrate at step 410. In other implementations, the device may select a nextlower transmission rate, as discussed above in connection with FIG. 2.If the rate exceeded the threshold, at step 435, the device may select anew transmission rate. The device may then attempt to retransmit thepacket at the new transmission rate. Selecting a new transmission ratemay comprise the device estimating the characteristic for a later timeperiod based on rate of change at step 433, and the device selecting afastest transmission rate of a plurality of transmission rates having acorresponding required value at step 434.

Still referring to FIG. 4, and in more detail, at step 405, the devicemay select an initial transmission (TX) rate. The device may also bereferred to as a first device. In some embodiments, the device may be anaccess point or a client device as described in FIGS. 1A and 1B. Theinitial TX rate may be a maximum TX rate of the device, which may have alower range or physical distance that the signal can be successfullytransmitted or received compared to a lower TX rate. The maximum TX ratemay be set based on the specifications of the device. In someembodiments, the device may select a preconfigured TX rate as theinitial TX rate. In some embodiments, the device may select a minimum TXrate as the initial TX rate, where the minimum TX rate is the lowest TXrate possible on the device. In other embodiments, the device maydetermine an initial transmission rate during establishment of theconnection via a rate negotiation mechanism.

At step 410, the device attempt to transmit data to a second device at aselected rate. For instance, after step 405, an initial TX rate may bethe selected TX rate. The second device may be referred to as arecipient device. In some embodiments where the device is an accesspoint, the access point may attempt to send a packet of data to a clientdevice at the initial TX rate. In some embodiments, the device may senda plurality of transmissions at the selected transmission rate. Theplurality of transmissions may be sent sequentially. In someembodiments, the transmission may comprise a transmission controlprotocol (TCP) synchronization (SYN) packet, while in other embodiments,the transmission may comprise a data packet.

At step 415, the device may determine whether the transmission to thesecond device has failed. In some embodiments, the device may determinewhether to retransmit the transmission. In some embodiments, the devicemay wait for a response, a reply, or an acknowledgment (ACK) from therecipient device after sending the transmission at the selected rate. Insome embodiments, the recipient device may be configured to send back anACK packet after receiving the transmission from the first device. Aftera predefined period of time, or a timeout period, the first device maynot receive a reply from the second device, indicating that thetransmission has failed. In some instances, the first device may receivea reply that has a signal to noise ratio or another channelcharacteristic below a predetermined threshold. In some instances, thefirst device may receive a reply from the recipient device indicatingthat the transmission has failed (e.g. a NACK packet, which may be sentresponsive to receipt of a packet having a signal to noise ratio orother characteristic below a threshold, responsive to a timeout of anexpected transmission period, or any other such determination). If thetransmission has not failed, then another transmission comprisinganother packet may be sent to the recipient device at the same selectedrate. In some embodiments, if the transmission has not failed, thenanother transmission comprising another packet may be sent to therecipient device at a higher transmission rate. In some embodimentswhere the device had sent a plurality of transmissions, a timeout periodcorresponding to the first of the plurality of transmission may haveelapsed, indicating that the first transmission has failed. In someembodiments, step 415 may be skipped, such as where transmission channelcharacteristics (e.g. noise, signal strength, etc.) are quicklydegrading, and the device may immediately select a lower transmissionrate at step 416 and retransmit the packet at 410. This may be helpfulin instances where the potential time to successfully transmit a packet(e.g. while the devices are within range) is less than a retransmissiontimeout period.

At step 416, if the transmission has failed, then the device mayidentify a relative velocity between the device and the second device.In some embodiments, the device may make assumptions about whether thedevice or the second device is stationary, and so the relative velocitymay be calculated with either the velocity of the device or the velocityof the second device. In some embodiments, the relative velocity may becalculated with both the velocity of the device and the second device.Location of the device and/or the second device may be identified tocalculate the relative velocity. In some embodiments, relative velocitymay be used to calculate a relative location of the device and thesecond device. In some embodiments, a relative location between thedevice and the second device may be used. In some embodiments, locationand/or velocity may comprising monitoring an acceleration of the deviceor the second device. In some embodiments, location and/or velocity maycomprise monitoring a change in position of the device or the seconddevice from an identified stationary position of the device or thesecond device. In some embodiments, the device or the second device mayuse a directional antenna instead of an omnidirectional antenna. Therelative location or the relative velocity may be used with thedirection of the directional antenna. For instance, the relativelocation may indicate that the second device will be within the rangeand direction of the directional antenna. In some embodiments, thecommunication channel between the devices may use beamforming, which maybe used to determine the relative velocity.

In some embodiments, the relative velocity and/or location may becalculated using sensors and/or location monitoring located within thedevice or the second device. For instance, the device and/or the seconddevice may comprise or connected to an accelerometer, global positionsystem (GPS), speedometer, altimeter, gravity sensors, gyroscopes,rotational vector sensors, orientation sensors, magnetometers and/orother sensors and systems. Location and/or velocity may be calculatedvia trilateration or triangulation.

In some embodiments, if the transmission rate selected by the device isbelow a minimum threshold, then the device may determine to not sendanother transmission. The minimum threshold may be preconfigured for thedevice. In some implementations, any transmission rates below theminimum threshold may only allow transmission of data packets that aresmaller than a standard size.

As discussed above, in some implementations of step 416, relativevelocity and/or positions of the devices may be determined throughexplicit determinations of location, acceleration, etc. In otherimplementations, at step 420, identifying a relative velocity may beperformed implicitly, and may comprise the device monitoring acharacteristic of a communication channel of the transmission. Thecharacteristic may comprise a minimum signal to noise ratio, minimumsignal strength of a transmission, or maximum block error rate. In someembodiments, the characteristic may comprise a maximum bit error rate, aminimum received signal strength indicator, a standing wave ratio, amaximum signal to interference and noise ratio, a signal to interferenceplus noise and distortion ratio, and/or any other such characteristic.The device may monitor one or more characteristic of the transmission byexamining, for example, a background noise or interference on aparticular frequency, determining a number of blocks or bits thatincludes errors, or determining a power level received by an antenna ofthe device. In other implementations, relatively velocity may bedetermined through a combination of explicit and implicit means. Forexample, a device may have access to its own location or positioninformation via a GPS sensor or accelerometer of the device, and maydetermine that a second device has a relative velocity implicitlythrough changing signal strength of the second device's transmissions.

In some embodiments, one or more characteristics of the communicationmay be stored in a memory location or cache of the device. In someembodiments, the characteristic may be stored in association with asecond device identifier, frequency, channel and/or time of thetransmission or reply. In some embodiments, a change in thecharacteristic may be stored in association with the communication. Insome implementations, memory may be limited by size such that only apredefined number of latest characteristics may be stored for eachclient device or second device. In some embodiments, a rate of change inthe characteristic may be stored in relation to the characteristic, bycomparing the measured characteristic with a previous characteristicstored in memory.

At step 430, in some embodiments, identifying a relative velocity maycomprise the device determining whether the characteristic is changingat a rate exceeding a threshold. The threshold may be specified in amemory or storage element of the device. The threshold may be definedsuch that minor variations in characteristics may be ignored whendetermining whether to retransmit a failed transmission using a newlower transmission rate. For instance, minor variations may be due toinherent errors due to sensor deviations, numerical errors, and/orsignal propagation variations (e.g. location of obstacles between thedevices). In some embodiments, the threshold may be defined such thatsmall movements in the devices do not affect the transmission rate. Forinstance, a mobile phone connected to a WiFi network may move severalfeet within a room. Although a characteristic may change, the accesspoint may keep using the same transmission rate. The threshold may be astatic value or it may be changed by a firmware and/or other software.The threshold may be stored in association with a different type ofcommunication channel. For instance, one channel may have one thresholdvalue, and another channel may have a different threshold value. If thecharacteristic is not changing at a rate exceeding the threshold, thepacket or data may be retransmitted to the second device using the sametransmission rate. If the characteristic is changing at a rate exceedingthe threshold, the devices may be moving away from each other and themethod may continue to step 435 of identifying a new transmission rate.For example, the characteristic may be a signal to noise ratio (SNR),and the change in signal to noise ratio may be −10 decibels (dB). Inother words, if the SNR drops by 10 dB, then the first device mayidentify a new transmission rate at step 435.

At step 435, the device may select a new transmission rate. In someembodiments, the new transmission rate may be lower than a previouslyused transmission rate. In some embodiments, the device may haveidentified a relative velocity between the device and the second deviceat 416, and the new transmission rate may be based on the relativevelocity. For instance, the relative velocity rate may be used toestimate the distance between the devices at a time when theretransmission will be sent at the next iteration of step 410, and thenew transmission rate may be selected such that a range corresponding tothe new transmission rate may be greater than or equal to the estimateddistance. In some embodiments, the relative velocity may be combinedwith a location of the device and/or the second device to calculate theestimated distance. In some embodiments, a memory element may include achange in transmission rate or the retransmission rate that correspondsto distance, estimated distance, and/or relative velocity.

As discussed above, received signal strength and signal to noise ratiosmay vary as a function of distance, and accordingly may also bedependent on relative velocity between two devices. As devices movepart, the distance between them may grow in a linear or non-linearfashion. For example, given a first device on a train on a straighttrack approaching, passing, and receding a platform, the distance fromthe first device to a second device on the platform will be initiallyvery large and will decrease relatively slowly. However, as the trainapproaches the platform, the distance will decrease to a lowest amountwhen the train is adjacent to the platform, with the change in distanceincreasing to a maximum relative velocity at the same point. Similarly,as the train recedes, the relative velocity will decrease as thedistance increases. Accordingly, a retransmission rate may depend onboth distance and relative velocity between the devices. In someimplementations, these distances may be measured directly, such as viaGPS or other location services. In other implementations, the distancesmay be estimated via received signal strength. Similarly, the relativevelocities may be estimated based on a rate of change of the measured orestimated distances. In still other embodiments, the distances and/orrelative velocities may be determined indirectly via a measurement of anacceleration of the first device, such as measured via an accelerometerof the device. The estimated distance and relative velocities may beused in selecting the new transmission rate from a plurality ofpotential transmission rates. The device may estimate the worst casedistance separation between the devices after a first transmission asfailed (since the retransmission timeout and transmission time isknown), such that the device may determine the correspondingretransmission rate that has the greatest chance of success and producean appropriate throughput.

In some embodiments, the new transmission rate may be selected based ona value of a characteristic or a rate of change in value ofcharacteristic of a communication channel between the devices. In someembodiments, the device may have determined that a characteristic of acommunication channel between the devices is changing at a rate greaterthan a threshold at step 430. In some embodiments, the new transmissionrate may be selected from a plurality of potential transmission rates,each of the plurality of potential transmission rates corresponding to arequired value for a characteristic of a communication channel betweenthe device and the second device. For instance, the plurality ofpotential transmission rates may be stored in a memory element, and eachof the potential transmission rates may be stored with a correspondingrequired value for the characteristic. For example, the memory elementmay store a table with entries of 10 dB corresponding to 1 megabits persecond (Mbit/s), 20 dB corresponding to 10 Mbit/s, 30 dB correspondingto 100 Mbit/s, and 40 dB corresponding to 300 Mbit/s. In someembodiments, the value of the characteristic may be used to calculatethe transmission rate (e.g. multiplying and adding constant values)instead of looking up corresponding values in a table. In someembodiments, the rate of change of value of the characteristic may beused to calculate a new transmission rate or the change in thetransmission rate. For instance, the SNR may have dropped by 10 dB persec (dB/s), and the transmission rate may be lowered by a correspondingamount, using a table stored in memory or by calculation from the drop.The table in memory may store, for instance, a drop in 10 dB/scorresponding to a drop in 30 Mbits/s of transmission rate. Those havingthe ordinary skill in the art will recognize that different units ofmeasurements may be used and that a device will have differentcapabilities of range and power of the signals it can produce.

At step 433, in some embodiments, selecting a new transmission rate mayinclude estimating a value of the characteristic for a later time periodbased on the rate of change. The device may estimate a value for themonitored characteristic at a time period subsequent to a retransmissionattempt, based on the rate of change of the monitored characteristic.For instance, the rate of change for the characteristic may be −10 dB/sand the current SNR may be 35 dB. The device may estimate that at a timeof retransmission (e.g. 1 second later), the SNR will likely be 25 dB.In some embodiments, the device may add the rate of change and thecurrent value of the characteristic. In some embodiments, the estimatemay take into account a time difference between retransmission time andthe time of when the value of the characteristic was measured. Forinstance, the retransmission time may be in 0.1 seconds, the rate ofchange may be −5 dB/s, and the current value of the characteristic maybe 35 dB. The estimate for the SNR at the next retransmission time maybe:

−5 dB/sec*0.1 sec+35 dB=34.5 dB

In some embodiments, a plurality of rates of change may be used toestimate the value of characteristic for a later time period. Forinstance, the last two rates of change may be averaged and combined withthe current value of the characteristic to estimate the value ofcharacteristic for a later time period. In some embodiments, multiplerates of change may be used to calculate a rate of change of the ratesof change.

At step 434, in embodiments that include estimating the characteristicfor a later time period, selecting a new transmission rate may includeselecting a fastest transmission rate of a plurality of transmissionrates having a corresponding required value for the characteristic lessstringent that the estimated value. For instance, the estimated valuemay be 34.5 dB for SNR. The plurality of transmission rates may include1 Mbit/s, 10 Mbit/s, 100 Mbit/s, and 300 Mbit/s, or any other suchvalues. Each transmission rate of the plurality of transmission ratesmay correspond to a required value for the characteristic. For instance,300 Mbit/s may correspond to 40 dB, 100 Mbit/s to 30 dB, 10 Mbit/s to 20dB, and 1 Mbit/s to 10 dB. For an estimated value of 34.5 dB, 100 Mbit/smay be selected because it corresponds to a required value of 30 dB forthe SNR that is less stringent than the estimated value. In anotherinstance, for an estimated value of 29.5 dB, 10 Mbit/s may be selected.The plurality of transmission rates may be stored in a memory table orbe calculated based on the estimated value. After a new transmissionrate is calculated, the device may attempt to retransmit the data orpacket to the second device at step 410.

FIG. 5 is a flowchart of an embodiment of a method of dynamicallyselecting transmission rate responsive to differences in transmissionrates. In brief overview, at step 505, a device may transmit to a seconddevice at a first rate having a first required value. At step 510, thedevice may determine whether to retransmit. If no retransmission isnecessary, then the method ends for that packet or data, and the nextpacket or data may be transmitted. If retransmission is necessarybecause the first attempt failed or if the device determines toretransmit the packet, then at step 515, the device retransmits at asecond rate having a second required value. At step 520, the devicedetermines whether another retransmission is necessary. Ifretransmission was successful, the method ends. If not, at step 525, thedevice selects a third rate. In some embodiments, a difference between athird required value of the third rate and the second required value maybe greater than a difference between the second required value and thefirst required value. In other embodiments, the third rate may beselected to be lower than the second rate by a predetermined amount,based on a predetermined multiplier, or may be a next lower rate in atable or list of predetermined transmission rates. At step 530, thedevice retransmits again at the selected third rate. Still referring toFIG. 5, and in more detail, at step 505, a first device may transmit, toa second device, a data packet at a first rate having a first requiredvalue for a transmission channel characteristic. In some embodiments,the first rate may be the maximum transmission rate of the device. Insome embodiments, the first rate may be an initial transmission rateused for new connections or new clients. In some embodiments, the firstrate may be a minimum transmission rate. The first rate may correspondto a first required value for a transmission channel characteristic. Thefirst rate and the corresponding first required value may be stored in amemory element of the first device. The transmission channelcharacteristic may be, for instance, a minimum SNR, a signal strength ofpackets received from the second device, a maximum block error rate, amaximum bit error rate, a minimum received signal strength indicator, astanding wave ratio, a maximum signal to interference and noise ratio, asignal to interference plus noise and distortion ratio, and/or any othersuch characteristic. For instance, the first rate may be 300 Mbit/swhich may correspond to a value of 40 dB for a characteristic of minimumSNR. In some embodiments, a timeout period may correspond to thetransmission, during which a response is expected to be sent from thesecond device to the first device (e.g. an ACK packet, a response,etc.). The first device may then wait for a response from the seconddevice, or send subsequent packets with the same first rate until aresponse is received or a timeout period associated with thetransmission is reached.

At step 510, the device may determine whether to retransmit the packet.In general, this step may be similar to step 415 described in relationto FIG. 4. The determination may be based on whether the firsttransmission at step 505 has failed. In some embodiments, detectingwhether the first transmission has failed may include detecting that aresponse, a reply, or an acknowledgement has not been received within atime out period corresponding to the first transmission. In other words,the determination to retransmit is based on an expiration of aretransmission time out period for the previous transmission of thepacket. The time out period may be based on a protocol used in thetransmission and/or specified in the data packet. In some embodiments,the determination to retransmit the packet is performed responsive tonot receiving a response to the packet within a predetermined timeperiod. In some embodiments, the predetermined time period may be lessthan the time out period. In other embodiments, the predetermined timeperiod may be longer than the time out period. In some embodiments, theshorter of the timeout period and the predetermined time period may beused to determine whether to retransmit the packet; in otherembodiments, the longer of the time out period and the predeterminedtime period may be used. In some embodiments, the predetermined timeperiod may be based on an average time to receive a response for one ormore previously transmitted packets.

In some embodiments, a response may be received by the first deviceindicating that the first transmission has failed. In some embodiments,a response received by the first device or the transmission sent by thefirst device may have a channel characteristic above or below the firstrequired value for the transmission channel characteristic thatcorrespond to the first rate. For instance, if the first rate was 300Mbit/s and corresponds to a value of 40 dB, the first device maydetermine to retransmit the data packet if the transmission channelcharacteristic is below 40 dB. If the first transmission has not failed,then the first device may not need to retransmit the packet to thesecond device. In other implementations, the first device may determineto retransmit the packet without waiting for expiration of a time outperiod. For example, in one such implementation, the first device mayimmediately retransmit the packet, responsive to a high packet loss ratefor previous packets. In another such implementation, the first devicemay retransmit the packet responsive to a channel characteristicdegrading faster than a retransmission timeout period. For example, if afirst packet with a time out period of 5 seconds is transmitted on achannel having a SNR of 50 dB, at a transmission rate requiring aminimum SNR of 30 dB, and one second later, the device detects that theSNR has degraded to 40 dB (−10 dB/second), the device may determine thatif the device waits for the full time out period to retransmit thepacket, the SNR of the channel may be at an unusable level. Accordingly,in such implementations, the device may retransmit the packet,responsive to the quickly degrading channel characteristic. In a furtherimplementation, the device may select a lower transmission rate andretransmit the packet, the rate selected based on the detected rate ofdegradation.

At step 515, if the first transmission has failed, the device mayretransmit the packet at a second rate having a second required valuefor the transmission channel characteristic. In some embodiments, thesecond rate may be lower than the first rate. In some embodiments, thesecond rate may be a predefined rate. In some embodiments, the secondrate may be determined as described in relation to FIG. 2. For instance,a lower transmission rate may be selected based on a stepwise function.In some embodiments, the second rate may correspond to a second requiredvalue for the transmission channel characteristic. In some embodiments,the second rate may be determined dynamically as described in relationto FIG. 4. For instance, the second rate may be determined based on therelative velocity between the first and the second device. The secondrate and the corresponding second required value may be stored in amemory element of the first device. In some embodiments, the first andsecond rates and the corresponding first and second required values maybe stored in the memory element of the first device. In someembodiments, a table may store all the rates and the correspondingrequired values. Each entry in the table may include a transmission rateand a corresponding require value. The table may be in descending orascending order of transmission rates. In some embodiments, each any twoadjacent entries of the table may include transmission rates that has asame interval as every other adjacent entries of the table. In otherwords, the entries in the table may be equally spaced out. In someembodiments, the second rate may have the same units of measurement asthe first rate. In some embodiments, the first required value may be inthe same units of measurement as the second required value; in otherembodiments, they may be different characteristics and/or have differentunits of measurements. For instance, the first required value may be fora SNR characteristic, while the second required value may be apercentage value that is a block error rate characteristic. Althoughprimarily discussed in terms of lower rates, in some implementations,the second rate may be a higher or faster transmission rate. Forexample, if channel characteristics are quickly improving (e.g. as adevice approaches the recipient device), then a faster retransmissionrate may be selected, responsive to the higher likelihood of successfulcommunication.

At step 520, the first device may determine whether to retransmit thedata packet again. This step may be similar to step 510. If theretransmission in step 515 has failed, or if the transmission channelcharacteristic is below the second value, the first device may attemptto transmit the data packet yet again. In some embodiments, thedetermination to retransmit is based on an expiration of aretransmission time out period for the previous transmission of thepacket. The time out period may be based on a protocol used in thetransmission and/or specified in the data packet, may be determinedduring a handshaking procedure, may be pre-configured by anadministrator, may be determined dynamically based on a packet lossrate, or any other source.

At step 525, the first device may select a third rate. The third ratemay be lower than the second rate and may be such that a differencebetween a third required value of the third rate for the transmissionchannel characteristic and the second required value is greater than adifference between the second required value and the first requiredvalue. For instance, the first transmission rate may be 300 Mbit/s andthe second transmission rate may be 250 Mbit/s. The difference betweenthe first transmission rate and the second transmission rate would be 50Mbit/s, and the third rate may be selected such that the differencebetween the third rate and the second rate is greater than 50 Mbit/s.For instance, the third rate may be 150 Mbit/s.

In some embodiments, the third rate may be selected from a table witheach entry including a transmission rate and a corresponding requiredvalue. The first and second rates may be included in the first twoentries of the table. In some embodiments, if the entries are equallyspaced out, the third rate may be selected from the fourth entry of thetable. In some embodiments, entries may not be equally spaced out, andthe difference between the first and second rates may be calculated. Thedifference may then be used with the second rate to find a transmissionrate threshold. The third rate may be selected from the table based onthe transmission rate threshold. For instance, the third rate may be arate that is the largest transmission rate below the transmission ratethreshold.

In some embodiments, the third rate may be selected responsive toidentifying, by the first device, a relative velocity between the firstdevice and second device exceeding a predetermined threshold. Forinstance, the relative velocity between the first device and the seconddevice may be identified as described in relation to step 416 of FIG. 4.In some embodiments, the predetermined threshold may indicated the rateat which the devices are moving away from each other. For instance, therelative velocity may be 10 meters for second (m/s), with a direction ofeast. The predetermined threshold may apply just to the magnitude of therelative velocity. In some embodiments, the third rate may be selectedresponsive to identifying a change in distance between the devicesexceeding a predetermined threshold. In some embodiments, the third ratemay be selected responsive to identifying a distance between the devicesexceeding a predetermined threshold. For instance, the distance betweenthe devices may exceed 100 meters, at which point a third rate may beselected. In some embodiments, selecting a third rate further comprisesselecting the third rate such that the difference between the thirdrequired value and the second required value is greater than adifference between the second required value and the first requiredvalue multiplied by a factor proportional to the relative velocity. Thefactor may be calculated from the relative velocity. In someembodiments, the factor may be the magnitude of the relative velocitymultiplied by any number between 0.01 and 100. The factor may then bemultiplied to the difference between the second required value and thefirst required value.

For example, the first required value may be 40 dB, and the secondrequired value may be 30 dB, for a difference of 10 dB. The factor,calculated based on the relative velocity or change in distance, may be1.2. The difference may be multiplied by the factor, resulting in aproduct of 12 dB. The third required value may thus be the secondrequired value minus the product, resulting in 18 dB. The thirdtransmission rate may correspond to the third required value orcalculated from the third required value. For instance, the thirdtransmission rate may be selected from a plurality transmission rates ina table that corresponds to a required value that is closest to, or isbounded by the third required value. For instance, the third requiredvalue may be 18 dB, and the entries in the table may correspond torequired values of 20 dB, 15 dB, and 10 dB. The entry that includes therequired value of 15 dB may be selected because it includes a largestrequired value less than the third required value. In other embodiments,the third required value may be used to calculated the transmissionrate.

In some embodiments, if the selected retransmission rate is lower than apredefined threshold, the first device may determine not to send aretransmission. In some embodiments, the predefined threshold may be setsuch that a data packet that is sent below the threshold may be smallerthan a standard packet size.

At step 530, after the third transmission rate has been selected, thefirst device may retransmit the data packet at the third transmissionrate. In some embodiments, if the retransmission at the third ratefails, the first device stops communication with the second device. Insome embodiments, if the retransmission at the third rate fails, theremay be another retransmission with a fourth transmission rate calculatedor selected similar to step 525. In some embodiments, if there was asuccessful transmission, another data packet may be transmitted with thetransmission rate that was used for the successful transmission. In someembodiments, another data packet may be transmitted with a newtransmission rate, where the new transmission rate may be selectedresponsive to the relative velocity or other channel characteristic ofthe successful transmission, similar to step 525. In some embodiments,if there was a successful transmission, the difference between the thirdrate and the second rate may be used to select a fourth transmissionrate to send the next data packet.

FIG. 6A is a graph depicting an embodiment of a timeline withtransmission rates using the dynamic transmission selection of FIG. 4.Similar to FIG. 3, when the distance between a first device and a seconddevice is changing, then the possible or allowable maximum TX rate willvary over time due to the varying distance between the devices. Forinstance at time 601, the two devices may start relatively close to oneanother, and thus the maximum allowable TX rate based on the distancemay be relatively high at 621. In some instances, the devices may moveaway from each other, and the maximum allowable TX rate may decrease: attime 602, for instance, the allowable TX rate may decrease to 622; attime 603, the allowable TX rate may yet again decrease to 623; and attime 602, the allowable TX rate may yet again decrease to 624.

Using the method described in relation to the flowchart of FIG. 4, attime 601, the first device, for example, an access point or mobiledevice, may select an initial TX rate 611 that may be greater than theallowable TX rate 621 at that time, as described in step 405 of FIG. 4.The transmission at the initial rate may fail, as described in step 415of FIG. 4. The first device may then identify a second TX rate 602 asdescribed in step 416 of FIG. 4. The second TX rate may be selected suchthat the second TX rate 612 will be lower than the allowable TX rate 622at time 602. As described in step 416, the TX rate may be selectedresponsive to the relative velocity between the two devices or by achannel characteristic of the transmission. At time 603, the firstdevice may select an even lower TX rate 613 such that it is lower thanthe allowable TX rate 623. Again, the lower TX rate may be selected asdescribed in step 416. At time 604, yet another TX rate may be selected.Because the selected TX rate is lower than the allowable TX rate attimes 602, 603 and 604, each transmission at those times may succeed. Insome embodiments, at time 602, a first data packet may be transmitted,and at time 603, a second data packet may be transmitted. Likewise, attime 604, a third data packet may be transmitted. In some instances, ifthe TX rate selected by the first device is below a minimum thresholdrate, then the first device may cease any further transmission with thesecond device.

FIG. 6B is a graph depicting an embodiment of a timeline withtransmission rates using the dynamic transmission selection of FIG. 5.As described in relation to FIG. 3 and FIG. 6A, the allowable TX rates621′, 622′, 623′, and 624′ may be limited by the distance between thetwo devices at different times of 601′, 602′, 603′, and 604′. At time601′, the first device may select a first TX rate 651 as described instep 505 of FIG. 5. The transmission may fail due to the first TX rate651 being greater than the allowable TX rate 621′ at time 601′. Thefirst device may select a second TX rate 652 as described in step 515.The second transmission may also fail because the second TX rate 652 maybe greater than the allowable TX rate 622′ at time 602′. The firstdevice may then select a third TX rate 653, as described in step 525.The third rate may be selected such that a second difference 662 betweenthe third rate and the second rate may be greater than a firstdifference 661 between the first rate and the second rate. The seconddifference 662 may be selected such that it is the first difference 661multiplied by a correction factor. The correction factor may bedetermined by the relative velocity between the devices or by a channelcharacteristic of the second and/or first transmission. Because theselected third TX rate 653 may be less than the allowable TX rate 623′,the third transmission may succeed. Upon a successful transmission, thefirst device may transmit a different data packet at time 604′. The TXrate 654 that the first device uses may be based on the previous TX rate653, the first difference 661, and/or the second difference 662. In someembodiments, the fourth transmission may be not sent if the selected TXrate 654 is less than a predefined threshold.

FIG. 6C is another graph illustrating an implementation of dynamic rateselection. A device may include a predetermined list or table oftransmission rates (and corresponding maximum signal to noise ratios forsuccessful transmission) as shown in the example table 680. The devicemay have a plurality of predetermined rates from a maximum to a minimumlevel, and may be at predetermined intervals or may have varyingintervals. As discussed above, in some implementations, upon determiningthat a transmission was unsuccessful (e.g. via a negativeacknowledgement or a retransmission timeout), a device may select a nextlower transmission rate. For example, given a first unsuccessfultransmission at 300 Mbps, the device may attempt retransmissions at thesubsequent rates of 270 Mbps, 240 Mbps, 180 Mbps, etc. However, suchrate changes may be too slow in instances where the relative velocitybetween devices is high or the distance between the devices is rapidlychanging.

For example, as shown, as a device (e.g. device B) moves away from asecond device (e.g. device A), the signal to noise ratio (SNR) 682 oftransmitted signal between the devices may vary, with the ratio growingsmaller as the distance increased from a first time (t1) through to alater, fourth time t4. If a first transmission at a first rate (e.g. 300Mbps) at time t1 fails, the device may attempt to perform aretransmission at time t2. If the device simply steps to the nexttransmission rate in the table (e.g. 270 Mbps), the required SNR may be21 dB, but the actual SNR may be 18 dB as shown, and accordingly, theretransmission will fail. Similarly, the device may attempt a secondretransmission at a next lower rate (e.g. 240 Mbps) with a required SNRof 19 dB, but the actual SNR at time t3 may be only 13 dB. Accordingly,when there is a high relative velocity between the devices, simplystepping through a table of transmission rates designed for relativelystationary devices may be inadequate or unsuccessful.

Instead, via the systems and methods discussed herein, the relativevelocity between the two devices may be estimated to predict a SNR atthe next retransmission time, and an appropriate transmission rate maybe selected for the retransmission. In one such implementation, theactual SNR between the devices may be measured periodically orcontinuously (e.g. by measuring received signal strength of datatransmissions or periodic pings or monitoring packets, in comparison tomeasured noise levels). Fluctuations or a rate of change of the SNR maybe determined. For example and using the curve 682 illustrated in FIG.6C, given a plurality of measurements of SNR before time t1 and/orbetween time t1 and t2, the device may project an estimated SNR at timet2 of 18 dB. If a transmission fails at time t1, then at time t2 whenthe device determines to perform a retransmission, the device may selectfrom the transmission rate table 680 a rate with a corresponding SNRrequirement below the estimated SNR, such as the 180 Mbps rate with thecorresponding 16 dB SNR requirement. Similarly, based on measurementsprior to t1, between t1 and t2, and/or between t2 and t3, the device maydetermine an estimated SNR at t3 of 13 dB. If a second retransmission isrequired, the device may select a transmission rate of 90 Mbps with acorresponding SNR requirement of 9 dB, less than the estimated 13 dB.

Thus, in some implementations, the device may select a retransmissionrate by estimating, based on changes in relative positions of the deviceand another device, relative velocities between the devices,accelerations of either device, and/or measured received signalstrengths or signal to noise ratios, an estimated signal to noise ratioof the connection at a retransmission time. The device may use theestimated signal to noise ratio to select a transmission rate having acorresponding required signal to noise ratio less than the estimatedsignal to noise ratio. In other implementations, other environmental orconnection characteristics may be utilized (e.g. signal strength, blockerror rate, etc.), and the table may include similar correspondingrequirements (e.g. required signal strength, required minimum blockerror rate, etc.) for each transmission rate.

FIG. 7 is a set of graphs depicting embodiments of timelines thatillustrate the relationship between time, distance to recipient device,signal to noise ratio, and maximum transmission rate possible. Graph 701shows the distance between a first device and a second device over timeas one device approaches and then passes the other. At time 710, thedistance between an access point and a client device may be relativelylarge compared to the distance at time 714. For example, the firstdevice may be stationary and the second device may be in a vehicle thatis approaching the first device, as in FIG. 1A. In another example, thefirst device may be approaching a stationary client device, or both thefirst device and a second device may be moving towards each other, as inFIG. 1B. As the devices approach each other, the distance between themdecreases, from time 710 to time 711, then to time 712, then to time713, and finally comes closest to each other at time 714. The devicesmay then move away from each other and the distance between the devicesmay increase from time 714, to time 715, to time 716, to time 717, andthen to time 718. Graph 702 shows signal to noise ratio (SNR) over timeand as the SNR relates to the distance between the devices. At time 710,SNR may be relatively small, due to the relatively large distancebetween the devices which may influence, for instance, interference,signal strength, and signal propagation. Over time, SNR may increase asthe distance between the devices decrease. SNR may increase from time710 to time 711, to time 712, to time 713, and to time 714. As distanceincreases, SNR may decrease from time 714 to time 715, to time 716, totime 717, and to time 718. Graph 703 shows TX rate possible over time,as it relates to SNR and distance. At time 710, due to a relatively lowSNR, a relatively low TX rate may be possible in a transmission betweenthe devices. As SNR increases due to the distance decreasing, the TXrate possible may increase over time. In some embodiments, the TX ratepossible may increase in discrete steps, due to the specifications ofthe first device. In some embodiments, the TX rate possible may increasecontinuously. From time 710 to time 711, to time 712, and to time 713,TX rate possible may increase. Between time 713, to time 714 and time715, the TX rate possible may be at a maximum TX rate for the firstdevice and/or the second device. After time 715, as the SNR decreasesand as the distance increase, the TX rate possible may decrease at time716, time 717 and time 718. In some embodiments, if a TX rate is below athreshold, or if SNR is below a threshold, no transmission is sent. Insome embodiments, the distance as graphed 701 or relative velocity maybe used to select a transmission rate for a retransmission. In someembodiments, SNR as graphed 702 or another characteristic of acommunication channel may be used to select a transmission rate for aretransmission. The selected retransmission rate may be less than the TXrate possible as graphed 703.

FIG. 8 is a block diagram illustrative of an embodiment of a system fordynamic transmission rate selection. In brief overview, the system maycomprise a device 800 in communication with one or more clients 850 a-nand a network 860. The device may comprise a wireless interface 805, aprocessor 820, a sensor 825, memory 830, and a network interface 840.The wireless interface 805 may comprise a TX rate control 810 and anantenna 815. The memory 830 may comprise TX rate selector 835.

Still referring to FIG. 8, the device 800 may be a wireless accesspoint. The device 800 may be a router, or a wireless router. The device800 and clients 850 a-850 n may be deployed as and/or executed on anytype and form of computing device, such as a computer, network device orappliance capable of communicating on any type and form of network andperforming the operations described herein. The device 800 may implementthe set of networking standards specified by IEEE 802.11, or any othertype and form of wireless communication standard. In otherimplementations, device 800 may be another client device, a desktopcomputer, a laptop computer, a server, a cellular tower, a gateway, aBluetooth device, a radio frequency identifier (RFID) source or reader,or any other device used for wireless communications and capable oftransmitting and receiving data at different rates.

The wireless interface 805 may comprise TX rate control 810 and antenna815. The wireless interface 805 may be a network interface. The TX ratecontrol 810 may tune the antenna 815 to a selected transmission rate.The antenna 815 may be capable of receiving and transmitting wirelesssignals of selected frequencies. In some embodiments, the wirelessinterface 805 may utilize a plurality of antennas and TX rate controls,for multiple-input and multiple-output wireless communication. In someembodiments, the antenna 815 may be tuned to a particular channel. Insome embodiments, the antenna 815 may communicate at frequencies of 2.4GHz, 3.6 GHz, 4.9 GHz, 5 GHz, 5.9 GHz, 60 GHz, or 900 MHz, or carrierfrequencies within bands centered on such frequencies. In someembodiments, the antenna 815 may communicate at a specified channelbandwidths, for instance, at 20 MHz, 40 MHz, 80 MHz, or 160 MHz. In someembodiments, the antenna 815 may be used to allow channel bonding. Insome embodiments, the antenna 815 may be used for beamforming or spatialfiltering. In some embodiments, the antenna 815 may comprise multipleantennas.

The wireless interface 805 may be configured to transmit a data packetfrom the device 800 to one or more of the clients 850 a-850 n. Thewireless interface 805 may transmit the data packet at a first rate, andit may also transmit the data packet at a second rate based on whetherthe first transmission has failed or responsive to other suchretransmission determinations, as discussed above.

The clients 850 a-850 n (generally referred to as 850) may be any typeof a computing device. For instance, a client 850 may be a mobiledevice, such as a laptop, tablet computer, smart phone, wearablecomputer, smart vehicle, or any other such device. In some embodiments,the client 850 may be an access point. A client 850 may have essentiallysame components as the device 800, such as a processor, memory, wirelessinterface, etc. The client 850 may be capable of receiving and sendingtransmissions from and to the device 800 through its own wirelessinterface.

The processor 820 is any logic circuitry that responds to and processesinstructions fetched from the main memory unit 830. The processor 820may also be referred to as central processing unit (CPU). The processor820 may be a microcontroller, a microprocessor, or a digital signalprocessor core. In some embodiments, the processor 820 comprisesmultiple cores. In some embodiments, the device 800 may comprise aplurality of processor 820. In some embodiments, the processors may bepart of a chipset. In some embodiments, the processor may be a multiplebit processor, such as a 16-bit, 32-bit, or a 64-bit processor. In someembodiments, the processor may be of a specific architecture, such as aMicroprocessor without Interlocked Pipeline Stages (MIPS) architecture,x86 architecture, or an ARM architecture. In some embodiments, theprocessor 820 may be part of a system-on-a-chip (SoC). In someembodiments, the device 800 may further include a graphical processingunit (GPU).

In some embodiments, the processor 820 may communicate with or includecache memory. The processor 820 may communicate directly with cachememory via a secondary bus, sometimes referred to as a backside bus. Inother embodiments, the processor 820 communicates with cache memoryusing a system bus. Cache memory typically has a faster response timethan main memory 830 and is typically provided by SRAM, BSRAM, or EDRAM.The processor 820 may communicate with various sensors 825, the wirelessinterface 805, and the network interface 840 via a local system bus,which may be a VESA VL bus, an ISA bus, an EISA bus, a MicroChannelArchitecture (MCA) bus, a PCI bus, a PCI-X bus, a PCI-Express bus, or aNuBus. In some embodiments, the processor 820 may use an AdvancedGraphics Port (AGP) to communicate with a display. In some embodiments,the processor 820 may communicate with the sensor 825, wirelessinterface 805, and/or the network interface 840 via HyperTransport,Rapid I/O, or InfiniBand.

Main memory unit 830 may be one or more memory chips capable of storingdata and allowing any storage location to be directly accessed by themicroprocessor 820, such as Static random access memory (SRAM), BurstSRAM or SynchBurst SRAM (BSRAM), Dynamic random access memory (DRAM),Fast Page Mode DRAM (FPM DRAM), Enhanced DRAM (EDRAM), Extended DataOutput RAM (EDO RAM), Extended Data Output DRAM (EDO DRAM), BurstExtended Data Output DRAM (BEDO DRAM), Enhanced DRAM (EDRAM),synchronous DRAM (SDRAM), JEDEC SRAM, PC100 SDRAM, Double Data RateSDRAM (DDR SDRAM), Enhanced SDRAM (ESDRAM), SyncLink DRAM (SLDRAM),Direct Rambus DRAM (DRDRAM), or Ferroelectric RAM (FRAM). In someembodiments, the memory 830 may comprise a content-addressable memory(CAM) or a ternary content-addressable memory (TCAM). The main memory830 may be based on any of the above described memory chips, or anyother available memory chips capable of operating as described herein.The processor 820 may communicate with main memory 830 via a system bus.In some embodiments, the processor 820 communicates directly with mainmemory 830 via a memory port. In some embodiments, the device 800 mayinclude one or more storage devices, such as a flash drive, a harddrive, a solid-state drive, an optical-disc drive and/or a USB drive.

The memory 830 may comprise a TX rate selector 835. The TX rate selectormay be configured to select a transmission rate based on previoustransmissions. For instance, if two earlier transmission have failed,the TX rate selector 835 may comprise code to take a difference oftransmission rates of the two earlier transmission and to select a newtransmission rate based on the difference. The TX rate selector 835 mayfurther communicate with the sensor 825 and/or receive information fromthe wireless interface 805 to identify a relative velocity between thedevice and the second device, and to select a transmission rate based onthe relative velocity. The TX rate selector 835 may further select atransmission rate based on a transmission channel characteristic ofearlier transmissions.

Furthermore, the computing device 800 may include a network interface840 to interface to a Local Area Network (LAN), Wide Area Network (WAN)or the Internet through a variety of connections including, but notlimited to, standard telephone lines, LAN or WAN links (e.g., 802.11,T1, T3, 56 kb, X.25), broadband connections (e.g., ISDN, Frame Relay,ATM), wireless connections, or some combination of any or all of theabove. The wireless interface 805 may also be a type of networkinterface 840, and the network interface 840 may be wireless. Thenetwork interface 840 may comprise a built-in network adapter, networkinterface card, PCMCIA network card, card bus network adapter, wirelessnetwork adapter, USB network adapter, modem or any other device suitablefor interfacing the computing device 800 to any type of network capableof communication and performing the operations described herein. Forinstance, the network interface 840 may be connected to a network 860such as the Internet.

The device 800 may comprise one or more sensors 825. The sensor 825 maybe an accelerator, a GPS, speedometer, altimeter, gravity sensor,gyroscope, rotational vector sensor, orientation sensor, and/ormagnetometer. The sensor 825 may measure one or more of speed, velocity,movement, location, rotation, and/or orientation. The sensor 825 mayrelay a measurement to the processor 820 or write directly to memory830.

The device 800 may display output and accept input. In some embodiments,the device 800 may send output signals and accept input signals throughthe network interface 840 or the wireless interface 805. A wide varietyof I/O devices may be present in the computing device 800. Input devicesinclude keyboards, mice, trackpads, trackballs, microphones,touch-enabled displays and drawing tablets. Output devices include videodisplays, speakers, and printers. The I/O devices may be controlled byan I/O controller. The I/O controller may control one or more I/Odevices such as a keyboard and a pointing device, e.g., a mouse oroptical pen. Furthermore, an I/O device may also provide storage and/oran installation medium for the computing device 800. In still otherembodiments, the computing device 800 may provide USB connections toreceive handheld USB storage devices.

In some embodiments, the computing device 800 may comprise or beconnected to multiple display devices, which each may be of the same ordifferent type and/or form. As such, any of the I/O devices 130 a-130 nand/or the I/O controller may comprise any type and/or form of suitablehardware, software, or combination of hardware and software to support,enable or provide for the connection and use of multiple display devicesby the computing device 800. For example, the computing device 100 mayinclude any type and/or form of video adapter, video card, driver,and/or library to interface, communicate, connect or otherwise use thedisplay devices. In one embodiment, a video adapter may comprisemultiple connectors to interface to multiple display devices. In otherembodiments, the computing device 800 may include multiple videoadapters, with each video adapter connected to one or more of thedisplay devices. In some embodiments, any portion of the operatingsystem of the computing device 800 may be configured for using multipledisplays. In other embodiments, one or more of the display devices maybe provided by one or more other computing devices connected to thecomputing device 800, for example, via a network 860. These embodimentsmay include any type of software designed and constructed to use anothercomputer's display device as a primary or a second display device forthe computing device 800. In some embodiments, the device 800 will senduser interface information via the network interface 840 or the wirelessinterface 805 that allows an administrator to set options on the device800 over a network 860. One ordinarily skilled in the art will recognizeand appreciate the various ways and embodiments that a computing device800 may be configured to have multiple display devices.

In further embodiments, an I/O device may be a bridge between the systembus and an external communication bus, such as a USB bus, an AppleDesktop Bus, an RS-232 serial connection, a SCSI bus, a FireWire bus, aFireWire 800 bus, an Ethernet bus, an AppleTalk bus, a Gigabit Ethernetbus, an Asynchronous Transfer Mode bus, a HIPPI bus, a Super HIPPI bus,a SerialPlus bus, a SCI/LAMP bus, a FibreChannel bus, or a SerialAttached small computer system interface bus.

The device 800 may comprise one or more operating systems, which controlscheduling of tasks and access to system resources. The computing device800 can be running any operating system such as any of the versions ofthe Microsoft® Windows operating systems, the different releases of theUnix and Linux operating systems, any version of the Mac OS® forMacintosh computers, any embedded operating system, any real-timeoperating system, any open source operating system, any proprietaryoperating system, any operating systems for mobile computing devices, orany other operating system capable of running on the computing deviceand performing the operations described herein. Typical operatingsystems include: WINDOWS NT, WINDOWS CE, WINDOWS 7, WINDOWS 8, andWINDOWS 10, all of which are manufactured by Microsoft Corporation ofRedmond, Wash.; MacOS, manufactured by Apple Computer of Cupertino,Calif.; OS/2, manufactured by International Business Machines of Armonk,N.Y.; and Linux, a freely-available operating system distributed byCanonical Ltd. of London, United Kingdom, or any type and/or form of aUnix operating system, among others. In some embodiments, the device 800may comprise an operating system designed for routers or wirelessrouters.

It should be understood that the systems described above may providemultiple ones of any or each of those components and these componentsmay be provided on either a standalone machine or, in some embodiments,on multiple machines in a distributed system. The systems and methodsdescribed above may be implemented as a method, apparatus or article ofmanufacture using programming and/or engineering techniques to producesoftware, firmware, hardware, or any combination thereof. In addition,the systems and methods described above may be provided as one or morecomputer-readable programs embodied on or in one or more articles ofmanufacture. The term “article of manufacture” as used herein isintended to encompass code or logic accessible from and embedded in oneor more computer-readable devices, firmware, programmable logic, memorydevices (e.g., EEPROMs, ROMs, PROMs, RAMs, SRAMs, etc.), hardware (e.g.,integrated circuit chip, Field Programmable Gate Array (FPGA),Application Specific Integrated Circuit (ASIC), etc.), electronicdevices, a computer readable non-volatile storage unit (e.g., CD-ROM,floppy disk, hard disk drive, etc.). The article of manufacture may beaccessible from a file server providing access to the computer-readableprograms via a network transmission line, wireless transmission media,signals propagating through space, radio waves, infrared signals, etc.The article of manufacture may be a flash memory card or a magnetictape. The article of manufacture includes hardware logic as well assoftware or programmable code embedded in a computer readable mediumthat is executed by a processor. In general, the computer-readableprograms may be implemented in any programming language, such as LISP,PERL, C, C++, C#, PROLOG, or in any byte code language such as JAVA. Thesoftware programs may be stored on or in one or more articles ofmanufacture as object code.

While various embodiments of the methods and systems have beendescribed, these embodiments are exemplary and in no way limit the scopeof the described methods or systems. Those having skill in the relevantart can effect changes to form and details of the described methods andsystems without departing from the broadest scope of the describedmethods and systems. Thus, the scope of the methods and systemsdescribed herein should not be limited by any of the exemplaryembodiments and should be defined in accordance with the accompanyingclaims and their equivalents.

What is claimed is:
 1. A method for wireless retransmission rateselection, comprising: transmitting, by a first device to a seconddevice, a data packet at a first rate; determining, by the first device,to retransmit the data packet; identifying, by the first device, arelative velocity between the first device and second device; andretransmitting the data packet, by the first device to the seconddevice, at a second rate selected responsive to the identified relativevelocity.
 2. The method of claim 1, further comprising selecting thesecond rate from a plurality of potential transmission rates, each ofthe plurality of potential transmission rates corresponding to arequired value for a characteristic of a communication channel betweenthe first device and the second device.
 3. The method of claim 2,wherein identifying the relative velocity between the first device andsecond device further comprises monitoring the characteristic of thecommunication channel between the first device and second device; anddetermining that a monitored characteristic is changing at a rateexceeding a predetermined threshold.
 4. The method of claim 3, whereinselecting the second rate from the plurality of potential transmissionrates further comprises estimating a value for the monitoredcharacteristic at a time period for a retransmission attempt, based onthe rate of change of the monitored characteristic; and selecting afastest transmission rate of the plurality of potential transmissionrates having a corresponding required value for the characteristic lessstringent than the estimated value.
 5. The method of claim 4, whereinestimating the value for the monitored characteristic further comprisesestimating the value for the monitored characteristic based on aplurality of measurements of the characteristic prior to transmittingthe data packet.
 6. The method of claim 2, wherein the characteristiccomprises a signal to noise ratio of the communication channel.
 7. Themethod of claim 2, wherein the characteristic comprises a minimum signalstrength of a transmission by the second device.
 8. The method of claim2, wherein the characteristic comprises a maximum block error rate ofthe communication channel.
 9. The method of claim 1, wherein identifyingthe relative velocity between the first device and second device furthercomprises monitoring an acceleration of the first device or a change inposition of the first device from an identified stationary position ofthe first device.
 10. A system for wireless retransmission rateselection, comprising: a first device comprising a wireless networkinterface and a processor executing a transmission rate selector;wherein the network interface is configured to transmit a data packetfrom the first device to a second device at a first rate, and retransmitthe data packet from the first device to the second device at a secondrate; and wherein the transmission rate selector is configured toidentify a relative velocity between the first device and second device,select the second rate responsive to the identified relative velocity.11. The system of claim 10, wherein the transmission rate selector isconfigured to select the second rate from a plurality of potentialtransmission rates, each of the plurality of potential transmissionrates corresponding to a required value for a characteristic of acommunication channel between the first device and the second device.12. The system of claim 11, wherein the transmission rate selector isconfigured to monitor the characteristic of the communication channelbetween the first device and second device; and determine that amonitored characteristic is changing at a rate exceeding a predeterminedthreshold.
 13. The system of claim 12, wherein the transmission rateselector is configured to estimate a value for the monitoredcharacteristic at a time period for a retransmission attempt, based onthe rate of change of the monitored characteristic; and select a fastesttransmission rate of the plurality of potential transmission rateshaving a corresponding required value for the characteristic lessstringent than the estimated value.
 14. The system of claim 13, whereinthe transmission rate selector is configured to estimate the value forthe monitored characteristic based on a plurality of measurements of thecharacteristic prior to transmitting the data packet.
 15. The system ofclaim 11, wherein the characteristic comprises a signal to noise ratioof the communication channel.
 16. The system of claim 11, wherein thecharacteristic comprises a minimum signal strength of a transmission bythe second device.
 17. The system of claim 11, wherein thecharacteristic comprises a maximum block error rate of the communicationchannel.
 18. The system of claim 10, wherein the transmission rateselector is configured to estimate monitor an acceleration of the firstdevice or a change in position of the first device from an identifiedstationary position of the first device.